Homogeneous material devoid of dynamic loading has been reliably fastened in the past. However, when homogeneity varies and when dynamic loading changes, confounding fastener issues arise. Skeletal scaffolding is one example of non-homogeneous structure subjected to dynamic loading.
The skeleton is comprised of osseous tissue. Compact bone is one form of osseous tissue characterized in greater density than the other osseous tissue, called cancellous bone, trabecular bone or spongy bone. The cancellous bone has a higher surface area than the compact bone but is softer, weaker and less stiff. When embedding a screw into bone having both characteristics, the holding power of the screw (e.g. pull out strength) is limited by the screw's ability to be retained in the cancellous bone, since it is the weakest component. Yet the fastener must still be designed to cut through the compact bone. Prior art buttress threads (the industry standard) cannot address the non-homogeneity of bone under dynamic loading.
One result is the prior art's failure to provide a reliable thread portion that engages the bone of the patient in a manner which retards its working loose. Bone is a remarkable structure which varies both in hardness and elasticity as a function of both age and location. Loads on the fastener must accommodate not only these constraints but also the dynamics of forces generated by the patient in daily activities. Prior art buttress threads cannot address these requirements, resulting in the fastener working loose or even pulling out.
As a consequence, the industry custom is to offer long cancellous screws having greater pitch density to increase holding. These screws typically have shafts totally or partially threaded along the length and threads of constant crest diameter along the major length (excluding the starting, distal end) and the shaft diameter equals the crest diameter. Pull out resistance is therefore limited by the volume of cancellous bone that resides between the screw teeth and its cohesion with adjacent cancellous tissue.
The buttress thread is presently the industry standard. Acme threads are sometimes used, but are merely buttress threads with the exposed sharp thread truncated at the apex. This does not cure buttress thread frailties. Unfortunately, buttress threads are suitable for use only where load forces on the fastener are applied in one direction. (Bhandari, Design of Machine Elements (2007), page 204). Where the load forces are multidirectional or not unidirectional and axial, failure can occur. One manifestation of buttress thread failure is “toggling” where the fastener works on the bone and enlarges the hole within which the fastener resides, resulting in failure.
Screw insertion time is a function of screw length and the medium's resistance to screw advancement. In an operatory, shorter procedures are believed to promulgate better results to the extent that asepsis correlates to shorter exposure by the patient to a procedure. To address this, a few patents have proposed dual start, dual thread fasteners having two differing crest diameters (“hi-low”), one for each thread. Because the industry still embraces buttress thread geometry, they have not solved the primary problems of embedment in non-homogeneous osseous tissue under multi-directional, dynamic loading. Please see exemplary U.S. Pat. Nos. 6,743,233, 5,743,914 and 5,720,766. Curiously these patented technologies have not been adopted by the industry.